The present invention relates generally to semiconductor device fabrication and, in particular, to device structures for a bipolar junction transistor, fabrication methods for a bipolar junction transistor, and design structures for a bipolar junction transistor.
Bipolar junction transistors are typically found in demanding types of integrated circuits, especially integrated circuits destined for high-frequency applications and high-power applications. One specific application for bipolar junction transistors is in radiofrequency integrated circuits (RFICs), which are found in wireless communications systems, power amplifiers in cellular telephones, and other varieties of high-speed integrated circuits. Bipolar junction transistors may also be combined with complementary metal-oxide-semiconductor (CMOS) field effect transistors in bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuits, which take advantage of the positive characteristics of both transistor types in the construction of the integrated circuit.
Bipolar junction transistors constitute three-terminal electronic devices constituted by three semiconductor regions, namely an emitter, a base, and a collector. Bipolar junction transistors may be fabricated using a single semiconductor material, such as silicon, with differently doped regions to define the terminals. A heterojunction bipolar junction transistor (HBT) utilizes multiple semiconductor materials for at least two of the terminals and, thereby, takes advantage of the divergent properties (e.g., bandgap) of the different semiconductor materials. An example of such multiple semiconductor materials is silicon germanium in combination with silicon.
An NPN bipolar junction transistor includes two regions of n-type semiconductor material constituting the emitter and collector, and a region of p-type semiconductor material sandwiched between the two regions of n-type semiconductor material to constitute the base. A PNP bipolar junction transistor has two regions of p-type semiconductor material constituting the emitter and collector, and a region of n-type semiconductor material sandwiched between two regions of p-type semiconductor material to constitute the base. Generally, the differing conductivity types of the emitter, base, and collector form a pair of p-n junctions, namely a collector-base junction and an emitter-base junction. A voltage applied across the emitter-base junction of a bipolar junction transistor controls the movement of charge carriers that produce charge flow between the collector and emitter regions of the bipolar junction transistor.
Improved device structures, fabrication methods, and design structures are needed that enhance the device performance of bipolar junction transistors.